IR documentation of procs/chans

- Several keywords in DSLX's scanner, including "carry", "next", "while", and "proc", don't appear in the document(https://google.github.io/xls/dslx_reference/). It would be better to provide some examples about the usage of these keywords in the DSLX program.

- The document of Chisel's IR, FIRRTL, not only provides a well-defined semantics specification but also provides a lot of examples to illustrate the usage. However, the concept of "channel" and "proc" in IR, are hard to understand without examples.